mnesia:first/1 and mnesia:next/2

[Chandrashekhar Mullaparthi]

Hello everyone,

I need functions like mnesia:first/1 and mnesia:next/2 (which can be safely
run in the context of a transaction) similar to mnesia:dirty_first/1 and
mnesia:dirty_next/2 - I've implemented them by copying the read/2 function
and modifying it as follows :-) It seems to work alright.

Has anyone felt the need for such functions...? Any comments on the
implementation? Wont work very well for fragmented tables but I guess I can
live with that.

cheers
Chandru

%% ----------------------------------
%% Following code added to mnesia.erl
%% ----------------------------------

first(Tab) ->
    first(Tab, read).

first(Tab, LockKind) ->
    case get(mnesia_activity_state) of
	{?DEFAULT_ACCESS, Tid, Ts} ->
	    first(Tid, Ts, Tab, LockKind);
	{Mod, Tid, Ts} ->
	    Mod:first(Tid, Ts, Tab, LockKind);
	_ ->
	    abort(no_transaction)
    end.

first(Tid, Ts, Tab, LockKind)
  when atom(Tab), Tab /= schema ->
      case element(1, Tid) of
	  ets ->
	      ?ets_first(Tab);
	  tid ->
	      Store = Ts#tidstore.store,
	      Key = ?ets_first(Tab),
	      LockItem = {record, Tab, Key},
	      case LockKind of
		  read ->
		      lock(LockItem, read);
		  write ->
		      lock(LockItem, write);
		  sticky_write ->
		      lock(LockItem, sticky);
		  _ ->
		      abort({bad_type, Tab, LockItem})
	      end,
	      Key;
	  Protocol ->
	      dirty_first(Tab)
      end; 
first(Tid, Ts, Tab, LockKind) ->
    abort({bad_type, Tab}).


next(Tab, Key) ->
    next(Tab, Key, read).

next(Tab, Key, LockKind) ->
    case get(mnesia_activity_state) of
	{?DEFAULT_ACCESS, Tid, Ts} ->
	    next(Tid, Ts, Tab, Key, LockKind);
	{Mod, Tid, Ts} ->
	    Mod:next(Tid, Ts, Tab, Key, LockKind);
	_ ->
	    abort(no_transaction)
    end.

next(Tid, Ts, Tab, Key, LockKind)
  when atom(Tab), Tab /= schema ->
      case element(1, Tid) of
	  ets ->
	      ?ets_next(Tab, Key);
	  tid ->
	      Store = Ts#tidstore.store,
	      Key_1 = ?ets_next(Tab, Key),
	      LockItem = {record, Tab, Key_1},
	      case LockKind of
		  read ->
		      lock(LockItem, read);
		  write ->
		      lock(LockItem, write);
		  sticky_write ->
		      lock(LockItem, sticky);
		  _ ->
		      abort({bad_type, Tab, LockItem})
	      end,
	      Key_1;
	  Protocol ->
	      dirty_next(Tab, Key)
      end; 
next(Tid, Ts, Tab, Key, LockKind) ->
    abort({bad_type, Tab}).



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